There are many medical and cosmetic treatments of skin that utilize heating a region of skin to be treated. Among these are hair removal, treatment of vascular lesions and skin rejuvenation. In these treatments, a volume of skin tissue under the skin to be treated is heated to a temperature that sufficiently high to achieve a desired effect, which is typically in the range of 45-60° C. One method that has been used for heating the epidermal and dermal layers of the skin is pulsed radio-frequency (RF) energy. In this method, electrodes are applied to the skin and an RF voltage pulse is applied across the electrodes. The properties of the voltage pulse are selected so as to generate an RF current pulse in the tissue to be treated that heats the tissue to the required temperature. For example, U.S. Pat. No. 6,749,626 discloses use of pulsed RF energy for inducing collagen formation in the dermis.
When an RF current pulse is used to heat a volume of skin tissue, the temperature of the tissue volume rises from body temperature to the required temperature within the duration of the pulse, which is typically less than 100 msec. The temperature of the tissue volume thus rises vary rapidly. Since the final temperature will actually depend on the electrical properties of the tissue volume which vary from individual to individual, the rapid rise in temperature of the tissue volume limits control of the tissue heating. Moreover, the rapid rise in temperature prevents the user from stopping the treatment should the tissue volume become overheated. Thus, using an RF pulse to heat the skin carries a risk of overheating the skin which could result in permanent scarring or other damage to the skin surface. Such damage to the skin includes, for example, a first degree or higher burn, blisters, or blood coagulation.